DE3855699T2 - A method of manufacturing a superconducting article, and apparatus and systems using the article - Google Patents

Description

Field of the invention

The present invention relates to a method for producing a superconducting article.

Background of the invention

Since the discovery of superconductivity in 1911, until recently, almost all known superconducting materials were elemental metals (e.g. Hg, the first known superconductor), metal alloys or intermetallic compounds (e.g. Nb3Ge, probably the material with the highest transition temperature Tc known before 1986).

Recently, superconductivity was discovered in a new class of materials, namely metal oxides. See, for example, J.G. Bednorz and K.A. Müller, Zeitschr. f. Physik B - Condensed Matter, Volume 64, 189 (1986), who report on superconductivity in lanthanum-barium-copper oxide.

The article in question led to a worldwide research activity which very quickly led to further significant progress. This progress has led to the discovery to date that compounds in the Y-Ba-Cu-O system can have superconductivity transition temperatures Tc above 77 K, i.e. above the boiling point of liquid N₂ (see e.g. MK Wu et al., Physical Review Letters, volume 58, page 908, March 2, 1987 and PH Hor et al., ibid, page 911). The progress also led to the identification of the material phase responsible for the observed high-temperature superconductivity and to the discovery of compounds and processing methods which in turn led to the production of bulk samples of an essentially single-phase material with a transition temperature Tc of more than 90 K. See e.g. the article by RJ Cava et al., Physical Review Letters, Volume 58(16), pages 1676-1679, which is incorporated herein by reference.

The excitement in the scientific and engineering community caused by recent advances in the study of superconductivity is due, at least in part, to the enormous potential technical significance of having materials that are superconductive at temperatures that do not require cooling with expensive liquid helium. Liquid nitrogen is generally considered to be one of the cheapest coolants, and achieving superconductivity at liquid nitrogen temperatures has been a long-sought goal that until recently seemed unattainable.

Although this goal has now been achieved, there are still limits that must be overcome before the new oxide high-temperature superconductors can be used in many technical applications. In particular, processes for forming high-temperature objects that also have a technically important shape or form still need to be developed. Among the shapes of technical importance are elongated bodies with a normally conductive metal coating, such as wires and strips.

State-of-the-art metallic superconductors have been manufactured in wire and strip form and used, for example, for superconducting magnets. It is known that superconducting wires and the like are almost invariably surrounded by a sheath made of a normally conducting metal, which, among other things, forms an alternative current path in the event of a local loss of superconductivity.

A general overview of potential Possible applications for superconductors can be found in the following two books, for example: BB Schwartz and S. Foner, editors, Superconductor Applications: Squids and Machines, Plenum Press, 1977, and S. Foner and BB Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications and Applications, Plenum Press 1981. Possible applications include power transmission or high-voltage lines, electrical machines and superconducting magnets, which are used for example for fusion generators, MHD generators, particle accelerators, levitation trains, magnetic separators and energy storage devices. Another application area is junction devices and detectors. It is to be expected that many of the above-mentioned applications and also other applications of superconductivity would benefit economically if superconducting materials with higher transition temperatures could be used instead of the previously mentioned materials with a relatively low transition temperature Tc.

U.S. Patent Application Serial No. 034,117 ('117), filed April 1, 1987, entitled "Apparatus and Systems Comprising a Clad Superconductive Oxide Body, and Method for Producing Such Body" by S. Jin et al., discloses a method for producing a superconducting oxide wire and other elongated articles having a sheath of a normally conducting material. The method comprises heat treating the sheathed elongated article. The relevant oxides lose oxygen at relatively high temperatures (such as are typically required for sintering) and absorb oxygen in an intermediate temperature range. The above-mentioned patent application discloses various methods for carrying out the heat treatment such that the oxygen content of the sintered oxide is in the range in which superconductivity occurs and that the oxide has a suitable crystal structure. In a In the proposed process, the normally conductive metal sheath surrounding the oxide powder is perforated at suitable intervals so that the ambient oxygen can come into contact with the powder (see '117, page 8).

Although '117 discloses various processes that can be used to make an elongated, coated superconducting oxide article, such as a wire or a ribbon, other simple processes for making such articles that reliably allow relatively free access of oxygen to the superconducting oxide during heat treatment are of considerable technical and commercial importance. The present application discloses such a process.

Definitions

The (Ba,Y) cuprate system used here is a class of oxides with the general formula Ba2-xM1-yXx+yCu₃O9-δ, where M is one of the elements Y, Eu or La and X is one or more possible elements other than Ba and M selected from the elements with atomic number 57-71, Sc, Ca and Sr. x+y is typically in the range between 0 and 1 (with Ba and M at least 50% unsubstituted). Typically 1.5 < δ < 2.5. A particularly preferred subclass of the (Ba,Y) cuprate system is 0 ≤ y ≤ 0.1, where the optional element X is one or more of the elements Ca, Sr, Lu and Sc. For further examples see D.W. Murphy et al. Physical Review Letters, Volume 58(18), Pages 1888-1890, (1987).

The (La, Ba) cuprate system is a class of oxides with the general formula La2-xMxCuO4-ε, where M represents one or more divalent metals (e.g. Ba, Sr, Ca), where x ≥ 0.05 and where 0 ≤ ε ≤ 0.5.

The "normally conductive" metal is defined here as a metal which does not become superconducting at temperatures of technical interest, i.e. typically at temperatures above 2 K.

Short description of the drawings

Fig. 1 and 2 show a schematic representation of a cross section through an exemplary intermediate product according to the invention or through a coated superconducting object made from this intermediate product; and

Fig. 3 shows a schematic representation of an exemplary device according to the invention, namely a superconducting electromagnet.

The invention

A method is disclosed for producing a superconducting article with a porous normal conducting metal casing, the superconducting material being a chemical compound, which is typically an oxide. The method allows relatively free access of the ambient atmosphere to the superconducting material during heat treatment of the article, so that the chemical composition (e.g. the oxygen content of the superconducting oxide) does not change or at least lies within predetermined limits associated with the occurrence of superconductivity in the chemical compound.

The process comprises forming an intermediate product comprising a normally conducting metal shell surrounding a certain amount of a superconducting chemical compound. It further comprises forming an article from the intermediate product by one or more shape-changing process steps (e.g. drawing through dies or die rings, rolling or other forms of shape-changing process steps) and heat treatment of the object. The process parameters for the heat treatment are chosen, among other things, so that a noticeable sintering of the superconducting material typically occurs. The material of the normally conductive metal casing is chosen so that the casing material surrounding the superconducting material is relatively porous at least during part of the heat treatment, so that a suitable ambient atmosphere has relatively free access to the superconducting material.

Although the method according to the invention is preferably applicable in connection with oxides that have a high transition temperature Tc, such as (Ba, Y) cuprates and (La, Ba) cuprates, it is also applicable to other superconducting chemical compounds with a high transition temperature Tc (e.g. nitrides, sulfides, hydrides, carbides, fluorides and chlorides) if such other compounds exist. To simplify the presentation, reference is often made here to "oxides", but this is not intended to limit the invention.

Fig. 1 shows a schematic representation of a cross section through an exemplary intermediate product according to the invention (a "preform") 10, wherein the superconducting material is provided with the reference numeral 11, the normally conducting metallic cladding material with the reference numeral 12 and with the reference numeral 13 a (removable) sheath. The superconducting material is typically in powder form, e.g. (Ba, Y) cuprate powder with the composition Ba₂YCu₃O₇ or a mixture of superconducting powder and metal powder, as disclosed in the American patent with the file number 046,825, which was filed on May 5, 1987 under the title "Superconductive Oxide Body Having Improved Properties" for S. Jin et al. The intermediate product typically consists of about 5 - 70% of the superconducting material.

The normally conductive metallic cladding material is also typically made of powder or other particles (e.g. flakes), although this is not a mandatory requirement. The normally conductive metallic cladding may, for example, consist of a tubular article comprising two (or possibly more) mixed components (e.g. Al and Ag particles) selected such that one component can be removed after completion of the shape-changing step and before heat treatment (e.g. by etching) without removing the other component.

The normal conducting metal is preferably selected from metals or alloys that are relatively inert to oxidation and relatively benign to the superconducting material. The first condition is satisfied by metals that exhibit self-limiting surface oxidation (although these metals are not preferred), while the last condition means that the presence of the normal conducting metal does not lead to a significant deterioration of the superconducting properties of the superconducting material during the heat treatment. Exemplary normal conducting metals are Au, Ag, Pt and Pd (with Ag currently being preferred). The normal conducting metallic cladding material may also comprise composite particles, i.e. particles having a first metal core (e.g. of nickel or stainless steel) surrounded by a second metal coating (e.g. of silver). The intermediate product typically consists of about 5 to 50 vol.% of the normally conductive metallic cladding material.

The presently preferred embodiments optionally include a removable sheath. However, the sheath may be omitted if the normally conductive metallic sheath has a sufficiently high structural Strength to withstand the required cross-sectional reduction process steps or the other shape-changing process steps without breaking the structure. This is the case, for example, when the normally conductive metallic encasing material, as described above, is a two-component tubular article. The encasing preferably consists of a relatively pliable material which is removable after completion of the cross-sectional reduction step and/or the other shape-changing steps (e.g. changing the cross-section or the coil winding). The encasing comprises, for example, Al, Mg, Sn, Zn or alloys of these materials. It can also comprise polymers, such as plasticized thermoplastic or a partially cured heat- or thermosetting polymer, or a composite material made from a mixture of metal and a polymer and/or ceramic powder. Typical removal methods are etching (eg for an aluminum jacket), pyrolysis (eg for a polymer jacket), melting (eg for a Sn jacket) or mechanical removal, eg cutting, pulling or breaking (eg for a polymer jacket or a jacket made of composite material). Any jacket present comprises between about 5 and 30 vol.% of the intermediate product.

In an exemplary preferred embodiment of the invention, the intermediate product comprises an outer casing (eg an Al tube), a tubular porous normally conducting metal object (eg compacted or easily sintered Ag powder) surrounded by the outer casing, and superconducting oxide powder which almost completely fills a bore in the tubular porous object. Such an intermediate product can be produced, for example, by inserting a dummy core rod axially into the aluminum tube and holding it concentrically in the tube. The annular space between The tube and the core rod are now filled with silver powder. The powder is then compacted and the arrangement is optionally heated so that the silver powder is slightly sintered. The core rod is then removed and the space is filled with oxide powder.

There are many alternative methods for producing the intermediate product. For example, a binder or a grease-like polymer can be mixed with the normal-conducting metal powder. These additives serve to reduce cold welding during cross-sectional reduction (e.g. during wire drawing). This cold welding can lead to blockage of a significant fraction of the pores, thereby impairing gas access to the superconducting material. The porous cladding can also comprise more than one normal-conducting metal, either mixed together or arranged separately in separate areas of the cladding.

The intermediate product is drawn or rolled to reduce its cross-section or its cross-section is otherwise modified in a known manner until the desired cross-section is achieved. Those skilled in the art will recognize that the mechanical properties of the various materials that make up the intermediate product must be taken into account in order to ensure satisfactory behavior during mechanical processing.

The mechanical processing of the intermediate product, which comprises both a superconducting powder core and a normally conducting metallic powder coating, is typically associated with a displacement of the particles in the core and the coating. If the two types of powder have substantially different coefficients of friction, it may be advantageous to adjust the relative coefficient of friction accordingly. This is typically achieved by adding a suitable lubricant to one or both of the powders. Examples of lubricants include graphite powder, grease, or other organic materials. As discussed above, the addition of lubricants can also be used to control the amount of cold welding during the reduction process. Although cold welding of the jacket material to a small extent is desirable to improve its integrity, excessive cold welding will result in an undesirable reduction in the porosity of the jacket.

After the reduction in cross-section is complete, the elongated article is typically formed into a desired shape (e.g., a coil) while the outer sheath is removed by a suitable process (e.g., etching). Fig. 2 shows a schematic cross-section through an exemplary elongated article, wherein the superconducting material is designated by the reference numeral 21, while the porous normally conducting metal sheath is designated by the reference numeral 22. The resulting article is now heated to sinter the superconducting oxide powder, to adjust the oxygen content of the oxide, and to produce the desired crystal phase in essentially conventional manner (see, e.g., RJ Cava, cited above). The article is heated, for example, in an oxygen-containing atmosphere (such as air, flowing oxygen, or oxygen under high pressure) to a temperature in the range between 500 and 1000°C. It is then kept at this temperature for 1 to 100 hours in contact with the atmosphere before being cooled relatively slowly (typically at a cooling rate of less than about 250ºC per hour, with no intermediate stops at intermediate temperatures) in continuous contact with the atmosphere. excluded) to a temperature in the range between 200 and 500ºC. The mixture is then cooled to room temperature.

The elongated superconductor with a normally conducting metal cladding thus produced can, if desired, be coated with any suitable material, such as a polymer or with a normally conducting metal, for example by spraying or immersing it in a suitable liquid or melt, or by vapor deposition. Such a coating is preferably non-porous. It is advantageously chosen so that it is substantially impermeable to oxygen, water vapor and other gases. Alternatively, the surface of the porous normally conducting metal cladding can be selectively melted (e.g. by means of a laser) and re-solidified to seal the outside of the porous cladding, but retaining the superconducting properties of the superconducting core.

Superconducting articles according to the invention can be used in a number of devices and systems. Fig. 3 shows a schematic representation of an exemplary device, namely a superconducting electromagnet 30, wherein the reference numerals 31 and 32 designate a coated superconducting article according to the invention and a tubular support body, respectively.

As already mentioned above, the invention is not only usable for the production of superconducting oxide objects with a normally conducting metal coating, but in principle also for the production of objects surrounded by a normally conducting metal coating made of another material which must be in contact with an atmospheric gas (e.g. O₂, CO₂, N₂, Cl₂, F₂) during the heat treatment.

Example I:

A 3.2 mm thick stainless steel rod was inserted into an aluminum tube (6.3 mm outside diameter and 0.76 mm wall thickness) with a sealed bottom and held in the center. The space between the tube and the rod was filled with silver powder with an average particle size of 8 µm. The rod was then removed and the resulting space was filled with oxide powder with the composition Ba₂YCu₃O₇ and an average particle size of 2.5 µm, prepared in the manner described by Cava et al. (op. cit.), and compressed. The open end of the intermediate product thus formed (the preform) was now sealed by folding and swaging. Up to this point, processing was carried out in air. The diameter of the preform was then reduced to 4.6 mm in the conventional manner by drawing through dies or die rings (four passes). The outer aluminium shell was then dissolved by placing the drawn article in a sodium hydroxide bath. The remaining elongated article had an outer diameter of about 3.2 mm. The article was then held in a flow of oxygen at 930ºC for 8 hours, cooled in the furnace to 600ºC, held at this temperature for 8 hours and then cooled in the furnace to 300ºC with oxygen flowing through the furnace each time. The heat-treated elongated article was then tested and found to be superconductive at 77 K. Its transition temperature Tc was about 93 K.

Example II:

A tubular article (6.3 mm outside diameter and 5.3 mm inside diameter) was prepared in a conventional manner by hot pressing (600ºC) from thoroughly mixed silver and aluminum powders (2:1 by volume, average particle size 76.2 and 508 µm (0.003 and 0.02 inches, respectively)). One end of the tubular The second end of the article was closed and powder (with an average particle size of 2.5 µm) with the composition Ba₂YCu₃O₇ was filled into the core. After closing the second end, the cross-section of the preform thus formed was changed by rolling to a 0.76 mm thick ribbon, which was immersed in a sodium hydroxide bath until the aluminum component of the normally conductive metallic sheath was dissolved. The article thus produced with a porous sheath was then heat treated in the manner already described in Example I. It was superconductive at 77 K.

Example III:

A superconducting wire was prepared in substantially the same manner as in Example I, except that the wire was wound helically around a mandrel prior to dissolving the aluminum sheath. The resulting coil was placed on a tubular support substantially as shown in Fig. 3.

Claims (14)

1. Method for producing a superconducting object surrounded by a normally conducting metal casing, comprising the following process steps: a) formation of an intermediate product comprising a normally conducting metal shell surrounding a certain amount of a superconducting material; b) forming the article from the intermediate product by one or more shape-changing process steps; and (c) heat treating the article in such a way that the superconducting material is appreciably sintered and that the chemical composition of the sintered superconducting material after completion of the heat treatment is within predetermined limits associated with the occurrence of superconductivity in the material; characterized, that at least during part of the process step c) the normal metallic encasing material surrounding the superconducting material is a porous material so that an ambient atmosphere can come into contact with the superconducting material at least during this part of the step c). 2. The method of claim 1, wherein the superconducting material comprises a superconducting oxide and the ambient atmosphere contains oxygen. 3. A method according to claim 2, wherein the normally conductive encapsulating material comprises a metal selected from the group consisting of Au, Ag, Pt and Pd. 4. The method of claim 2, wherein the article is an elongated body and the method comprises providing the superconducting material with (Ba,Y)-cuprate powder or (La,Ba)-cuprate powder, and wherein the normal conducting metallic cladding material comprises Ag. 5. The method according to claim 1, wherein the method step a) comprises the following substeps: (i) creating or providing a substantially tubular outer shell and an inner member which is disposed in the bore of the outer shell so as to define a generally annular space therebetween; ii) filling the annular space with a normally conductive metal powder; and iii) removing the inner part so as to form a substantially cylindrical space surrounded by the porous normal conducting metal, and filling the cylindrical space with a superconducting oxide powder; and the removal of the outer jacket takes place before performing process step c). 6. The method according to claim 1, wherein the normally conductive metallic encasing material comprises a material with a predetermined porosity prior to carrying out the method step c), and wherein the porosity of this material is increased prior to the completion of the method step c). 7. The method of claim 6, wherein the material having the predetermined porosity consists essentially of intermixed particles of at least a first and a second material, and wherein the increase the porosity of the material is achieved by bringing the object into contact with a selectively acting etching medium, by which the first material is etched away much faster than the second material. 8. The method of claim 7, wherein the first material particles are aluminum particles and the second material particles are silver particles. 9. The method of claim 1, wherein the shape-changing step comprises a step of reducing the cross-section, wherein the object is an elongated body, and wherein the elongated body is brought into a predetermined shape prior to completion of step c). 10. The method of claim 1, wherein the normally conductive metallic cladding material comprises composite material particles consisting essentially of a first metallic core and a second metallic coating surrounding the core. 11. Process according to one of claims 1 to 5 and 9, wherein a (Ba,Y) cuprate powder with the nominal composition Ba₂YCu₃O₇ is used, wherein the normally conductive cladding material consists essentially of silver powder, wherein the elongated body is also brought into a predetermined shape before the end of the process step c), and wherein the elongated body is brought to a temperature in the range between 500 and 1000°C for 1 to 100 hours during the process step c) and is cooled relatively slowly to a temperature in the range between 200 and 500°C. 12. The method according to claim 1, wherein the object is treated after carrying out step c) such that contact of the ambient atmosphere with the superconducting material is essentially prevented. 13. The method of claim 12, wherein treating the article comprises applying a coating to the article. 14. The method of claim 12, wherein treating the article comprises melting the outer portion of the normally conductive metallic sheath.